A spindle head for performing friction stir welding includes concentric spindle shafts driven by stacked, coaxial motors contained within a spindle housing. The coaxial arrangement of the motors results in a more compact package. Each of the motors is concentrically arranged around one of the spindles by directly connecting a rotor of the motor to a spindle shaft. The stators of the motors are mounted on the housing and are concentrically arranged around the concentric spindle shafts.